function remSize() {
    var deviceWidth = document.documentElement.clientWidth || window.innerWidth
    if(deviceWidth >= 750) {
        deviceWidth = 750
    }
    else if(deviceWidth <= 320) {
        deviceWidth = 320
    }
    document.documentElement.style.fontSize = (deviceWidth / 7.5) + "px"
}

remSize()

window.onresize = function(){
    remSize()
}